Luminescent fiber, production thereof and use thereof

ABSTRACT

Luminescent fiber comprising at least one fiber-forming material with at least one inorganic luminophor pigment dispersed therein, wherein the inorganic luminophor pigment has an average particle size of 1 to 30 μm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT Application No.PCT/EP99/00430, filed Jan. 25, 1999, which claims the benefit of GermanNational Application No. DE 19802588.2 which was filed Jan. 23, 1998.

This invention relates to luminescent fiber comprising at least onefiber-forming material having at least one inorganic luminophor pigmentdispersed therein.

Fibers which are luminescent because they have been dyed with solubleorganic ultraviolet-active dyes are known. Such fibers, which are basedon polyamide or viscose, are used for example in documents of value toprove their authenticity. These fibers emit green, yellow, red or bluelight on stimulation by UV radiation, depending on the dye or dyemixture used. However, such organically dyed fibers have thedisadvantage of low lightfastness, and organic ultraviolet-active dyesprovide neither infrared luminescence nor phosphorescence. Because ofthese disadvantages, such fibers have no utility for the securitymarking of documents of value.

DE-A-19 539 315 describes luminescent regenerated cellulose fibercontaining inorganic luminophors having an average particle size of lessthan 1 μm, especially of 0.5 to 0.7 μm, in an amount of 0.01 to 5% byweight. These inorganic luminophors are phosphates, tungstates, oxides,silicates and aluminates of the alkaline earth metals, of the transitiongroup elements or of the rare earths or halides of the alkali oralkaline earth metals, each doped with manganese, antimony, tin, lead,copper, silver or rare earths. These inorganic luminophors are superiorto organic dyes with regard to lightfastness, but produce neitherphosphorescence nor infrared luminescence effects and lead only to lowluminescence intensities, which may lead to uncertain results, forexample, in the examination of documents of value marked with suchluminophors.

It is an object of the present invention to provide luminescent fiber,especially for use in documents of value, which provides fluorescence orphosphorescence effects, possesses high lightfastness and exhibitsintensive luminescence effects in the visible and/or. nonvisiblewavelength range.

This object is achieved according to the invention by luminescent fibercomprising at least one fiber-forming material with at least oneinorganic luminophor pigment dispersed therein, when the inorganicluminophor pigment has an average particle size of 1 to 30 μm.

The intensification of luminescence through the choice of certainaverage particle sizes for the luminophor pigment is surprising, sinceDE-A-19 539 315 states expressly that the inorganic luminophors shouldadvantageously have an average particle size of less than 1 μm and inthe discussion of the background art cites the Japanese patentpublication No. 87-327 866 which says that yttrium oxysulfide luminophorpigment loses its luminescence effect on grinding to particle sizes ofless than 1 μm.

The choice of fiber-forming material in the invention is not subject toany specific restriction, save that it be miscible with the luminophorpigments of the claimed particle size. The fiber-forming material in theluminescent fiber of the invention is preferably viscose, since thuslyproduced luminescent fiber is highly compatible with the customary paperstocks based on cellulose and printable by various printing processes,such as offset, so that there are no problems with using such fiber formarking papers, specifically documents of value. The fiber of theinvention may also be used in the textile sector, for the covert orovert identification of high value branded products. The intensiveluminescence attainable together with high lightfastness makes theluminescent fiber of the invention particularly useful for the securitymarking of any fiber-containing articles, particularly textiles, papersand particularly documents of value.

The inorganic luminophor pigments used according to the invention havean average particle size of preferably 2 to 30 μm, more preferably 4 to20 μm, especially 5 to 20 μm. The use level in luminescent fiber ofluminophor pigments used according to the invention can vary within widelimits and is advantageously in the range from 0.01 to 50% by weight,preferably more than 5% by weight to 50% by weight, based on thewater-free total fiber mass. Preference is given to percentages rangingfrom 7 to 40%, especially from 10 to 20%, by weight, based on thewater-free total fiber mass.

Inorganic luminophor pigments particularly preferred for use in theinvention produce a phosphorescence effect on excitation by visible orultraviolet radiation. For luminophor pigments to produce aphosphorescence effect their luminescence must involve energy-storingprocesses, so that the luminescence is at least partly still presentafter the excitation has ended. The phosphorescence effect has theadvantage of being a simple way to ensure machine readability and ofpermitting the separation in space of the site of excitation from thesite of detection. The phosphorescence effect may be excited even bywhite light, so that visual observation in a darkened environment issufficient for detection. This facilitates the checking of any securitycoding of products, such as textiles, and the checking of documents ofvalue.

The invention advantageously utilizes inorganic luminophor pigmentswhich on excitation by visible or ultraviolet radiation in thewavelength range from 200 to 680 nm will, after the excitation hasended, emit visible light having spectral fractions in the wavelengthrange from 380 to 680 nm.

It is particularly advantageous to use zinc sulfides, zinc cadmiumsulfides, alkaline earth metal aluminates, alkaline earth metal sulfidesor alkaline earth metal silicates, all doped with one or more transitionmetal elements or lanthanoid elements. For instance, copper-doped zincsulfides produce green phosphorescence, alkaline earth metal aluminates,alkaline earth metal sulfides or alkaline earth metal silicates dopedwith lanthanoid elements produce green, blue or red phosphorescence, andcopper-doped zinc cadmium sulfides produce yellow, orange or redphosphorescence, depending on the cadmium content.

Preference is given to alkaline earth metal aluminates doped witheuropium and alkaline earth metal aluminates which, as well as europium,include a further rare earth element as coactivator, especiallydysprosium. Particularly useful alkaline earth metal aluminates of theabovementioned kind are described in EP-A-0 622 440 and U.S. Pat. No.5,376,303, which are both incorporated herein in full by reference.

Another group of inorganic luminophor pigments useful in the inventionproduce a fluorescence effect on excitation by ultraviolet radiation.These pigments emit green, yellow or blue light on stimulation byultraviolet radiation. The presence of UV-active luminescence-capablematerials is verifiable using simple commercial UV light sources. Thismay constitute an advantage in use, but in other cases, such asdocuments of value, constitutes a disadvantage, since the presence of asecurity feature is readily apparent to unauthorized persons.

Advantageous luminophor pigments among this group of luminophor pigmentsemit visible light having spectral fractions in the wavelength rangefrom 380 to 680 nm on excitation by ultraviolet radiation in thewavelength range from 200 to 380 nm. Particularly advantageous suchluminophor pigments are zinc sulfides, oxides, oxysulfides, silicates oraluminates, all doped with one or more transition metal elements orlanthanoid elements. For instance, copper-doped zinc sulfides producegreen fluorescence, silver-doped zinc sulfides blue fluorescence, andoxides, oxysulfides, silicates or aluminates doped with transitionmetals or lanthanoids produce green, blue or red fluorescence.

A further preferred group of luminophor pigments to be used according tothe invention is the group of the infrared-active luminophor pigments,ie pigments comprising infrared-active luminophors having a luminescenceprocess involving at least partly long-wave, infrared radiation havingwavelengths above 680 nm. These include not only the anti-Stokesluminophors, which on excitation by infrared radiation above 680 nm emitluminescence radiation having a shorter wavelength below 680 nm, butalso the Stokes luminophors, which on excitation by radiation of acomparatively short wavelength below 680 nm emit infrared radiationhaving a wavelength above 680 nm. Such anti-Stokes luminophors aredescribed inter alia in WO 98/39392, likewise incorporated herein infull by reference, and the prior art cited therein. This group ofluminophor pigments also includes infrared-infrared convertingluminophors, which on excitation by infrared radiation emit infraredradiation of a different wavelength.

The process for producing the luminescent fiber of the invention issimple in that it merely requires that the activator-doped inorganicluminophor pigment be added to the fiber-forming material or a solutionthereof and fiber be spun therefrom. For example, the doped pigment isadded to viscose dope, and the viscose dope is spun into fiber by theviscose spinning process. It is similarly possible to add the dopedpigments to a cellulose solution and spin fibers therefrom, for exampleaccording to the cupro process, the lyocell process or according to aprocess involving low-substituted cellulose ethers. An example of auseful solvent is N-methylmorpholine oxide/water.

Combined with a suitable source of excitation, such viscose fiber can beused for the security marking of products, for authentication and forcontrolling automatic recognition processes for textiles, documents ofvalue and security papers in the widest sense. It is necessary in thisconnection to use materials and security features that are difficult toforge and are producible in combination with other security features. Aswell as providing for simple visual inspection by anyone, it is alsodesirable that, if necessary and depending on the level of security,security features be only clearly detectable by a sophisticatedanalytical effort. Luminescent fiber permits a localized, high signalintensity and hence a better signal-to-noise ratio compared with uniformapplications of corresponding luminophor pigments by means of customarysecurity printing technologies. The long-term objective in theproduction of security features is the technical scientific lead overthe forger and the reduced motivation to forge because of complexityassociated with the security feature.

The fiber of this invention offers great application benefits withregard to simple, rapid, contactless and economical verification,machine readability, compatibility with other effects, specificexcitation by different wavelengths and the various luminescence colorson excitation by different wavelengths. The materials of the inventionare useful for producing uncopyable textiles, documents of value andsecurity papers, are highly compatible with the raw materials based oncellulose and are printable by various printing processes, specificallydie stamping and offset printing, providing for possible combinationswith security printing inks or with other security features.

Combined with a suitable source of excitation, the luminescent viscosefiber of the invention that provides a phosphorescence effect is forexample useful for the overt security marking of products and theirauthentication. The phosphorescence-effect fiber of the invention offerssubstantial benefits for this application with regard to simple, rapid,contactless and economical verification by anyone, since the necessaryexcitation of the phosphorescence effect is possible with just naturalor artificial white light and verification of the security feature ispossible just by visual inspection in a darkened environment. The use ofa photodetector, moreover, provides for simple machine readability inthat the phosphorescence effect also permits the site of excitation tobe separated in space from the site of verification.

The inventive fiber which provides a fluorescence but no phosphorescenceeffect likewise permits inspection by anyone, but the UV excitationneeded for detection is sufficient to increase the difficulty ofdetection and therefore constitutes a higher security level.

The use of infrared-active luminophors for marking and forgeryproofingdocuments of value constitutes a still higher security level in that thelow signal intensity of infrared-active luminophor pigments verysubstantially increases the analytical effort required for excitationand verification and makes it difficult even to detect the existence ofa corresponding security device.

EXAMPLE 1

A spinning solution containing 9 parts by weight of cellulose is admixedat room temperature with a dispersion of a luminophor pigment formed bystirring 100 parts by weight of a zinc sulfide having an averageparticle size of 5 μm and doped with 400 ppm of copper into a solutionof polyvinyl alcohol, water and Orotan wetting agent to form adispersion composed of 22 parts by weight of the monodisperse luminophorpigment, 2.8 parts by weight of polyvinyl alcohol and 0.05 part byweight of Orotan. The dispersion is filtered through a commerciallyavailable sieve having a mesh size of 40 μm, mixed with 10,000 parts byweight of spinning solution and spun under plant-customary conditionsinto an acidic coagulation bath. The fiber obtained on drying has agreen luminescence color on excitation by ultraviolet radiation.

EXAMPLE 2

Example 1 is repeated by mixing 100 parts by weight of a zinc sulfidehaving an average particle size of 5 μm and doped with 400 ppm of copperdirectly with 10,000 parts by weight of spinning solution, filteringthrough a commercially available sieve having a mesh size of 40 μm andspinning under plant-customary conditions into an acidic coagulationbath. The fiber obtained on drying has a green luminescence color onexcitation by ultraviolet radiation.

EXAMPLE 3

Example 1 is repeated by mixing 225 parts by weight of a zinc sulfidehaving an average particle size of 5 μm and doped with 400 ppm of copperin the form of a dispersion with 10,000 parts by weight of spinningsolution and spinning under plant-customary conditions into an acidiccoagulation bath. The fiber obtained on drying has a green luminescencecolor of higher intensity than Example 1 on excitation by ultravioletradiation.

EXAMPLE 4

Example 1 is repeated by mixing 100 parts by weight of a zinc sulfidehaving an average particle size of 20 μm and doped with 80 ppm of copperand 5 ppm of cobalt in the form of a dispersion with 10,000 parts byweight of spinning solution and spinning under plant-customaryconditions into an acidic coagulation bath. The fiber obtained on dryinghas a green luminescence color on excitation by ultraviolet radiationand produces green phosphorescence on excitation by white light.

EXAMPLE 5

Example 1 is repeated by mixing 100 parts by weight of an yttrium oxidesulfide having an average particle size of 10 μm and doped with 12% ofytterbium and 8% of erbium in the form of a dispersion with 10,000 partsby weight of spinning solution and spinning under plant-customaryconditions into an acidic coagulation bath. The fiber obtained on dryinghas a green luminescence color on excitation by infrared radiation.

What is claimed is:
 1. A fiber-containing article comprising aluminescent fiber wherein the luminescent fiber comprises at least onefiber-forming material with at least one inorganic luminophor pigmentdispersed therein, characterized in that the inorganic luminophorpigment has an average particle size of 1 to 30 μm.
 2. The fibercontaining article of claim 1, wherein the article is a document ofvalue.
 3. The fiber containing article according to claim 1, wherein thefiber-forming material is viscose, the inorganic luminophor pigment hasan average particle size of about 5 to 20 microns, and wherein theinorganic luminophor pigment is present in an amount of more than 5 to50%, by weight, based on the water-free total fiber mass.
 4. Afiber-containing article comprising a luminescent fiber comprising atleast one fiber-forming material and at least one inorganic luminophorpigment dispersed therein, under conditions effective to cause saidfiber to display phosphorescent or fluorescent luminescence and theinorganic luminophor pigment having an average particle size of fromabout 1 to about 30 microns.